Method for producing acrylic acid

ABSTRACT

There is provided a novel process for producing acrylic acid by which acrylic acid can be obtained from a raw material independent of petroleum. The process for producing acrylic acid according to the present invention comprises the steps of: applying a dehydration reaction to glycerol as a raw material in a gas phase; and then applying a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction.

This is a division of U.S. patent application Ser. No. 10/585,793 filedJul. 12, 2006 now U.S. Pat. No. 7,612,230 and claims the benefit thereofunder 35 U.S.C. §120, which was the U.S. national phase of InternationalApplication No. PCT/JP2005/001627, filed 28 Jan. 2005, which claimspriority from Japanese Patent Application No. 2004-024181 filed Jan. 30,2004, all of which applications (the International Application and theJapanese Application) are hereby incorporated by reference in theirentireties.

TECHNICAL FIELD

The present invention relates to a novel process for producing acrylicacid by which acrylic acid can be obtained from a raw materialindependent of petroleum.

BACKGROUND ART

As processes for producing acrylic acid, from old times there have beenknown a process in which acetylene, carbon monoxide and water are madeto react together in the presence of a nickel catalyst (so-called Reppeprocess) and a process in which acrylonitrile is hydrolyzed. However,thereafter, a process of two-step gas phase oxidation of propylene,namely, a process in which propylene is air-oxidized to convert it intoacrolein and then this acrolein is further oxidized to convert it intoacrylic acid, has been developed and now is industrially commonlyemployed (see, for example, JP-A-01-063543 (Kokai) and JP-A-55-102536(Kokai)).

DISCLOSURE OF THE INVENTION

Object Of The Invention

However, the conventional process of two-step gas phase oxidation ofpropylene uses petroleum-derived propylene as a raw material.Accordingly, in order to suppress global warming due to an increasingatmospheric level of CO₂ concentration and exhaustion of undergroundresources that are currently in progress, a process for producingacrylic acid from a raw material independent of petroleum is requested.Such a production process has, however, not yet been reported.

Thus, an object of the present invention is to provide a novel processfor producing acrylic acid by which acrylic acid can be obtained from araw material independent of petroleum.

Summary Of The Invention

The present inventors diligently studied to solve the above problems,and have consequently conceived a production of acrylic acid fromglycerol as a raw material that is easily available from oils-and-fatsexisting in the animate nature. Then, the present inventors havecompleted the present invention by finding that acrylic acid can beobtained by applying a dehydration reaction to glycerol in a gas phaseand then applying a gas phase oxidation reaction to a gaseous reactionproduct formed by the dehydration reaction.

That is, a process for producing acrylic acid according to the presentinvention is a process comprising the steps of: applying a dehydrationreaction to glycerol as a raw material in a gas phase; and then applyinga gas phase oxidation reaction to a gaseous reaction product formed bythe dehydration reaction.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, detailed descriptions are given about the process forproducing acrylic acid according to the present invention. However, thescope of the present invention is not bound to these descriptions. Andother than the following illustrations can also be carried out in theform of appropriate modifications of the following illustrations withinthe scope not departing from the spirit of the present invention.

The process for producing acrylic acid according to the presentinvention is a process comprising the steps of: applying a dehydrationreaction to glycerol in a gas phase; and then applying a gas phaseoxidation reaction to a gaseous reaction product formed by thedehydration reaction.

The present invention uses glycerol as a raw material. The glycerol tobe subjected to the dehydration reaction as a raw material may be 100%pure glycerol, or may be an aqueous glycerol solution which is a mixtureof glycerol and water. Glycerol and the aqueous glycerol solution arerecovered in hydrolysis of various oils-and-fats or from waste fluids insoap production and are therefore industrially easily available. Inaddition, glycerol is expected to be generated in abundance as aby-product in production of a bio-diesel fuel as a renewable fuel byhydrolysis of vegetable oils in the future, and effective use thereof isdesired. The present invention provides a process for producing acrylicacid as a useful chemical from glycerol which is such an easilyavailable and renewable raw material, and this process is also a processfor providing acrylic acid of which the source of carbon is carbondioxide fixed by plants and which substantially does not lead to theincrease of carbon dioxide in the air even when incinerated.

In the present invention, when the aqueous glycerol solution is used asa raw material, the water content of this aqueous glycerol solution ispreferably not more than 50% by weight. When the water content of theaqueous glycerol solution as a raw material is more than 50% by weight,the aqueous glycerol solution needs much energy to vaporize and alsoenormous cost of wastewater treatment, and is therefore sodisadvantageous economically as to hamper the industrial implementationof the process for producing acrylic acid of the present invention. Thewater content of the aqueous glycerol solution as a raw material is morepreferably not more than 30% by weight, still more preferably not morethan 20% by weight, and most preferably not more than 10% by weight.

The dehydration reaction in the present invention is a reaction ofconverting glycerol into acrolein and conducted by vaporizing the rawmaterial (glycerol or aqueous glycerol solution) so as to be gaseous andthen making this gas conduct a gas phase reaction in the presence of acatalyst. Examples of the catalyst that can be used in theaforementioned dehydration reaction include: natural and synthetic claycompounds such as kaolinite, bentonite, montmorillonite and zeolite;catalyst such that phosphoric acid or sulfuric acid is supported on asupport such as alumina; inorganic oxides or inorganic composite oxidessuch as Al₂O₃, TiO₂, ZrO₂, SnO₂, V₂O₅, SiO₂—Al₂O₃, SiO₂—TiO₂ andTiO₂—WO₃; solid acidic substances such as sulfates, carbonates, nitratesand phosphates of metals such as MgSO₄, Al₂(SO₄)₃, K₂SO₄, AlPO₄, andZr₃(PO₄)₂. It is usually preferable that these are used in the shape ofsuch as sphere, pillar, ring, saddle. When the above-mentioned substanceis a powder, it may be molded alone, or may be used in the form such asimpregnated into a already-molded support or applied to its surface. Thereaction temperature in the aforementioned dehydration reaction ispreferably set in the range of 200 to 370° C., though not especiallylimited.

Specifically, for example, the aforementioned dehydration reaction maybe conducted by vaporizing the raw material so as to be gaseous and thenpassing a flow of this gas through a reactor filled with theaforementioned catalyst and controlled to the aforementioned reactiontemperature. The flow rate of the gas when being passed through thereactor is, for example, preferably controlled to a space velocity of100 to 20000 h⁻¹, though not especially limited.

In addition, in the aforementioned dehydration reaction, it ispreferable that: to the gas generated by the vaporization of the rawmaterial, there is added an inert gas, and then a flow of the resultantmixed gas is passed through the reactor. Specifically, it is preferablethat the concentration of the inert gas in the gas being supplied to thereactor of the aforementioned dehydration reaction is controlled to notless than 50% by volume. Incidentally, as the aforementioned inert gas,for example, it is possible to use such as nitrogen gas, carbon dioxidegas, rare gas, and water vapor.

The gas phase oxidation reaction in the present invention is a reactionof converting acrolein, formed by the aforementioned dehydrationreaction, into acrylic acid and conducted by making a gaseous reactionproduct, formed by the aforementioned dehydration reaction, conduct agas phase reaction in the presence of a catalyst. Examples of thecatalyst that can be used in the aforementioned gas phase oxidationreaction include: solid catalysts including such as iron oxide,molybdenum oxide, titanium oxide, vanadium oxide, tungsten oxide,antimony oxide, tin oxide, copper oxide, and their mixtures andcomposite oxides. These may be used also in the form supported onsupports (e.g. zirconia, silica, alumina and their composite oxides, andsilicon carbide). The reaction temperature in the aforementioned gasphase oxidation reaction is preferably set in the range of 200 to 400°C., though not especially limited.

Specifically, for example, the aforementioned gas phase oxidationreaction may be conducted by passing a flow of the gas, formed by theaforementioned dehydration reaction, through a reactor filled with theaforementioned catalyst and controlled to the aforementioned reactiontemperature. The flow rate of the gas when being passed through thereactor is, for example, preferably controlled to a space velocity of100 to 2000 h⁻¹, though not especially limited.

In addition, in the aforementioned gas phase oxidation reaction, it ispreferable that: when a flow of the gas, formed by the aforementioneddehydration reaction, is passed through the reactor, oxygen gas isbeforehand added to that gas to increase the oxygen concentration. Bythis operation, the reactivity of the oxidation reaction is enhanced, sothat acrylic acid can be obtained in a higher yield. Specifically, it ispreferable that the oxygen concentration in the gas being supplied tothe reactor of the aforementioned gas phase oxidation reaction iscontrolled to not less than 2% by volume. Incidentally, if the oxygenconcentration in the gas being supplied to the reactor of theaforementioned gas phase oxidation reaction is too much high, then thereoccurs an unfavorable possibility that it may fall within the combustionrange to thus involve risks of such as explosion. Therefore it ispreferable that the upper limit value of the oxygen concentration isappropriately determined so as to avoid the combustion range inconsideration of such as the concentration of unreacted raw glycerol,contained in the gas formed by the dehydration reaction, and thereaction temperature.

The process for producing acrylic acid of the present invention has onlyto be a process which comprises the steps of: applying theaforementioned dehydration reaction to the aforementioned raw material;and then applying the aforementioned gas phase oxidation reaction to agaseous reaction product formed by the dehydration reaction. Thus, itsspecific working mode is not especially limited. For example, thefollowing modes can be adopted: i) a mode that there is used atandem-type reactor comprising two reaction tubes linked to each other,where the two reaction tubes are filled with a catalyst for thedehydration reaction and a catalyst for the gas phase oxidation reactionrespectively and where the dehydration reaction and the gas phaseoxidation reaction are separately conducted in their respective reactiontubes; ii) a mode that there is used a single-type reactor comprisingone reaction tube, where the reaction tube is filled with a catalyst forthe gas phase oxidation on the reaction gas outlet side and with acatalyst for the dehydration reaction of glycerol on the reaction gasinlet side, thus conducting in the one reaction tube the dehydrationreaction followed by the gas phase oxidation reaction; and iii) a modethat there is used a single-type reactor comprising one reaction tube,where the one reaction tube is filled with catalysts for the dehydrationreaction and for the gas phase oxidation reaction uniformly mixedtogether or with a catalyst which functions both for the dehydrationreaction and the gas phase oxidation reaction, thus conducting thedehydration reaction and the gas phase oxidation reaction in the onereaction tube at the same time.

The aforementioned mode of conducting the dehydration reaction and thegas phase oxidation reaction in the tandem-type reactor (theaforementioned i)) has an advantage that the reaction temperatures ofthose reactions can be individually controlled within their respectiveoptimum ranges, although there is an unfavorable possibility that thereaction product may crystallize in the joint of the reaction tubes tothus cause clogging. Moreover, in the tandem-type reactor, since a gascan be added through the joint, there is an advantage that oxygen can beadded into a gas transferring from the aforementioned dehydrationreaction to the aforementioned gas phase oxidation reaction, so it ispossible to take the aforementioned mode of increasing the oxygenconcentration in the gas which is to be subjected to the aforementionedgas phase oxidation reaction. Accordingly, in the aforementioned mode ofconducting the dehydration reaction and the gas phase oxidation reactionin the tandem-type reactor, it is preferable to add oxygen into the gastransferring from the aforementioned dehydration reaction to theaforementioned gas phase oxidation reaction.

The aforementioned modes of conducting the dehydration reaction and thegas phase oxidation reaction in the single-type reactor (theaforementioned ii) and iii)) have an advantage of enabling a compactsystem, although it is impossible to take the aforementioned mode ofincreasing the oxygen concentration in the gas which is to be subjectedto the aforementioned gas phase oxidation reaction or although there isa disadvantage in the maintenance aspect such as when only either one ofthe catalytic functions for the dehydration reaction and for the gasphase oxidation reaction is deteriorated, so that it becomes necessaryto replace the deteriorated catalyst. In addition, particularly, theaforementioned mode ii) has a further advantage that if necessary,conducting the dehydration reaction and the gas phase oxidation reactionat different reaction temperatures becomes possible, for example, bydividing a heat medium circulating part of a multitubular reactor intotwo parts and circulating heat media of different temperatures.Furthermore, in the aforementioned mode ii), by filling an inert supportbetween the catalyst for the gas phase oxidation and the catalyst forthe dehydration reaction, it is possible that mutual contaminations ofthe catalyst for the gas phase oxidation and the catalyst for thedehydration reaction are prevented, or that a catalytic reaction at animproper reaction temperature is suppressed. As the inert support, lowsurface area heat-resistant materials, for example, metal fillers (e.g.stainless Raschig rings) and ceramic sinters, can be used.

Incidentally, acrylic acid produced by the production process of thepresent invention can be industrially further subjected to publiclyknown purification methods (for example, processed by steps such as astep of collecting acrylic acid as a solution by using water or asolvent, a distillation step for removing low- and high-boiling-pointmaterials from the resultant solution containing acrylic acid, or acrystallization step for purifying acrylic acid by crystallizing it) tothus provide acrylic acid as a product. Then, this product can be usedto produce, for example, polyacrylic acids (salts) as water-solublepolymers or water-absorbent resins, by publicly known polymerizationmethods such as thermal polymerization methods and photopolymerizationmethods.

According to the present invention, acrylic acid can be obtained fromglycerol which is a raw material independent of petroleum. Further,according to the present invention, glycerol, which is a by-product inhydrolysis of vegetable oils for such as production of a bio-diesel fueland in soap production, can be effectively used as a raw material.Moreover, glycerol derived from vegetable oils is such that its carbonis derived from carbon dioxide in the air. Therefore, even if acrylicacid produced by the present invention and products therefrom arefinally incinerated, it does not lead to the increase of carbon dioxidein the air, and thus there is also an effect of preventing globalwarming. Besides, there is no worry of exhaustion of resources such asfossil resources, because vegetable oils are renewable resources.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is more specifically illustrated bythe following Examples of some preferred embodiments in comparison withComparative Examples not according to the present invention. However,the present invention is not limited to them. Hereinafter, unlessotherwise noted, the unit “% by mol” is referred to as “%”.

PRODUCTION EXAMPLE 1 Production of Catalyst (A1)

An amount of 194 g of a support in a spherical shape of 7 to 10 mmcomprising α-alumina as the main component was impregnated with 4 g ofphosphoric acid and 2 g of silica sol and then dried at 80° C. in arotary evaporator to obtain a catalyst (A1) of 2% by weight insupporting ratio of phosphoric acid.

PRODUCTION EXAMPLE 2 Production of Catalyst (A2)

Firstly, while 500 mL of water was heated at 90° C. and stirred, intothis there were added 63 g of ammonium paramolybdate, 19.2 g of ammoniummetavanadate and 8.0 g of ammonium paratungstate to dissolve them.Furthermore, an aqueous copper nitrate solution, which had beenpreviously prepared by dissolving 15.8 g of copper nitrate into 50 mL ofwater, was added to prepare a chemical liquid. Into a porcelainevaporating dish as set on a water bath of 90° C., there was placed 200g of a support in a spherical shape of 6 to 8 mm comprising α-alumina asthe main component, and then there was poured the aforementionedchemical liquid to support it thereon under stirring, thus obtaining acatalyst precursor. Next, this catalyst precursor was calcined for 6hours at 400° C. to obtain a catalyst (A2) of 31% by weight insupporting ratio of an Mo—V—W—Cu-composite oxide.

PRODUCTION EXAMPLE 3 Production of Catalyst (B1)

An amount of 188 g of a support in a spherical shape of 3 to 5 mmcomprising α-alumina as the main component was impregnated with 10 g ofphosphoric acid and 2 g of silica sol and then dried at 80° C. in arotary evaporator to obtain a catalyst (B1) of 5% by weight insupporting ratio of phosphoric acid.

PRODUCTION EXAMPLE 4 Production of Catalyst (B2)

Firstly, while 500 mL of water was heated at 90° C. and stirred, intothis there were added 70 g of ammonium paramolybdate, 21.3 g of ammoniummetavanadate and 8.9 g of ammonium paratungstate to dissolve them.Furthermore, an aqueous copper nitrate solution, which had beenpreviously prepared by dissolving 17.6 g of copper nitrate into 150 mLof water, was added to prepare a chemical liquid. Into a porcelainevaporating dish as set on a water bath of 90° C., there was placed 200g of a support in a spherical shape of 3 to 5 mm comprising α-alumina asthe main component, and then there was poured the aforementionedchemical liquid to support it thereon under stirring, thus obtaining acatalyst precursor. Next, this catalyst precursor was calcined for 6hours at 400° C. to obtain a catalyst (B2) of 35% by weight insupporting ratio of an Mo—V—W—Cu-composite oxide.

PRODUCTION EXAMPLE 5 Production of Catalyst (C)

An amount of 177 g of a support in a spherical shape of 3 to 5 mmcomprising α-alumina as the main component was impregnated with 20 g ofphosphoric acid and 3 g of silica sol and then dried at 80° C. in arotary evaporator to obtain a catalyst (C′) of 10% by weight insupporting ratio of phosphoric acid.

Next, while 500 mL of water was heated at 90° C. and stirred, into thisthere were added 70 g of ammonium paramolybdate, 21.3 g of ammoniummetavanadate and 8.9 g of ammonium paratungstate to dissolve them.Furthermore, an aqueous copper nitrate solution, which had beenpreviously prepared by dissolving 17.6 g of copper nitrate into 150 mLof water, was added to prepare a chemical liquid. This chemical liquidwas sprayed onto the surface of the aforementioned catalyst (C′)uniformly with a spray nozzle so that the chemical liquid componentwould be supported only on the outer portion of the catalyst, thusobtaining a catalyst precursor. Next, this catalyst precursor wascalcined for 6 hours at 400° C. to obtain a catalyst (C) of 32% byweight in supporting ratio of an Mo—V—W—Cu-composite oxide.

EXAMPLE 1

A tandem-type reactor comprising two reaction tubes linked to each other(both were made of SUS and had a diameter of 25 mm) was used. Thefirst-step reaction tube was filled with 50 mL of catalyst (A1), and thesecond-step reaction tube was filled with 50 mL of catalyst (A2). Then,those reaction tubes were placed into molten salt baths of variabletemperatures, and the molten salt temperatures were set to be 295° C.for the first step and 275° C. for the second step.

An aqueous glycerol solution having a water content of 15% by weight wasvaporized, and an oxygen-containing gas was added thereto. A flow of theresultant mixed gas (gas composition: glycerol 10% by volume, water 9%by volume, oxygen 6% by volume and nitrogen 75% by volume) was passedthrough the aforementioned first-step reaction tube and theaforementioned second-step reaction tube in sequence at a flow rate of420 mL/min, and then the discharged gas was collected with a collectingbottle containing water to obtain acrylic acid. Its yield was 55%.

EXAMPLE 2

A tandem-type reactor comprising two reaction tubes linked to each other(both were made of SUS and had a diameter of 25 mm) was used. Thefirst-step reaction tube was filled with 35 mL of catalyst (B1), and thesecond-step reaction tube was filled with 50 mL of catalyst (B2). Then,those reaction tubes were placed into molten salt baths of variabletemperatures, and the molten salt temperatures were set to be 290° C.for the first step and 270° C. for the second step.

An aqueous glycerol solution having a water content of 9% by weight wasvaporized, and nitrogen gas was added thereto. A flow of the resultantmixed gas (gas composition: glycerol 14% by volume, water 7% by volumeand nitrogen 79% by volume) was passed through the aforementionedfirst-step reaction tube at a flow rate of 294 mL/min, and then into agas flow having passed through the first-step reaction tube, there wasadded and mixed air at a flow rate of 126 mL/min, and then a flow of theresultant mixed gas was passed through the aforementioned second-stepreaction tube, and then the discharged gas was collected with acollecting bottle containing water to obtain acrylic acid. Its yield was63%.

EXAMPLE 3

A single-type reactor comprising one reaction tube (made of SUS andhaving a diameter of 25 mm) was used. This reaction tube was filled with50 mL of catalyst (B1), 10 mL of stainless Raschig rings, and 50 mL ofcatalyst (B2) in this order. The part filled with the catalyst (B1) wasused as a first-step reaction zone, and the part filled with thecatalyst (B2) was used as a second-step reaction zone. Then, theaforementioned reaction tube was placed into a molten salt bath whichwas segmented by a shield and was temperature-variable in each segment,wherein the shield was positioned so as to correspond to the filledposition of the stainless Raschig rings in the reaction tube so that thefirst-step reaction zone and the second-step reaction zone could beindependently controlled to their respective temperatures. The moltensalt temperatures were set to be 292° C. for the first step and 270° C.for the second step.

An aqueous glycerol solution having a water content of 8% by weight wasvaporized, and an oxygen-containing gas was added thereto. A flow of theresultant mixed gas (gas composition: glycerol 10% by volume, water 4%by volume, oxygen 6% by volume and nitrogen 80% by volume) was passedthrough the aforementioned reaction tube at a flow rate of 420 mL/min,and then the discharged gas was collected with a collecting bottlecontaining water to obtain acrylic acid. Its yield was 65%.

EXAMPLE 4

A single-type reactor comprising one reaction tube (made of SUS andhaving a diameter of 25 mm) was used. This reaction tube was filled with70 mL of catalyst (B1) and 30 mL of catalyst (B2) having been uniformlymixed together. Then, the aforementioned reaction tube was placed into amolten salt bath of variable temperature, and the molten salttemperature was set to be 295° C.

An aqueous glycerol solution having a water content of 2% by weight wasvaporized, and an oxygen-containing gas was added thereto. A flow of theresultant mixed gas (gas composition: glycerol 10% by volume, water 1%by volume, oxygen 6% by volume and nitrogen 83% by volume) was passedthrough the aforementioned reaction tube at a flow rate of 420 mL/min,and then the discharged gas was collected with a collecting bottlecontaining water to obtain acrylic acid. Its yield was 57%.

EXAMPLE 5

A single-type reactor comprising one reaction tube (made of SUS andhaving a diameter of 25 mm) was used. This reaction tube was filled with50 mL of catalyst (C). Then, the aforementioned reaction tube was placedinto a molten salt bath of variable temperature, and the molten salttemperature was set to be 294° C.

An aqueous glycerol solution having a water content of 9% by weight wasvaporized, and an oxygen-containing gas was added thereto. A flow of theresultant mixed gas (gas composition: glycerol 10% by volume, water 5%by volume, oxygen 6% by volume and nitrogen 79% by volume) was passedthrough the aforementioned reaction tube at a flow rate of 420 mL/min,and then the discharged gas was collected with a collecting bottlecontaining water to obtain acrylic acid. Its yield was 58%.

Industrial Application

The process for producing acrylic acid according to the presentinvention is a novel process by which acrylic acid can be obtained froma raw material independent of petroleum, and is useful as anext-generation process for producing acrylic acid. In addition, even ifacrylic acid produced by the process for producing acrylic acidaccording to the present invention, and products therefrom, are finallyincinerated, it does not lead to the increase of carbon dioxide in theair, and thus there is also an effect of preventing global warming.

1. A process for producing acrylic acid, comprising the steps of: a)vaporizing a raw material comprising an aqueous glycerol solution togenerate a first gas, wherein said aqueous glycerol solution has a watercontent of not more than 50% by weight; b) applying a dehydrationreaction to glycerol in a gas phase that includes said first gas; thenc) applying a gas phase oxidation reaction to a gaseous reaction productformed by the dehydration reaction to obtain said acrylic acid; and d)wherein the dehydration reaction and the gas phase oxidation reactionare conducted in a tandem-type reactor comprising the two reaction tubeswhich are linked to each other, where the two reaction tubes are filledwith a catalyst for the dehydration reaction and a catalyst for the gasphase oxidation reaction respectively, and where the dehydrationreaction and the gas phase oxidation reaction are separately conductedin their respective tubes.
 2. The process for producing acrylic acidaccording to claim 1, wherein oxygen is added to a gas which istransferred from the dehydration reaction to the gas phase oxidationreaction.
 3. A process for producing acrylic acid, comprising the stepsof: a) vaporizing a raw material comprising an aqueous glycerol solutionto generate a first gas, wherein said aqueous glycerol solution has awater content of not more than 50% by weight; b) applying a dehydrationreaction to glycerol in a gas phase that includes said first gas; thenc) applying a gas phase oxidation reaction to a gaseous reaction productformed by the dehydration reaction to obtain said acrylic acid; and d)wherein the dehydration reaction and the gas phase oxidation reactionare conducted in a single-type reactor comprising one reaction tube,where said one reaction tube is filled with a catalyst for the gas phaseoxidation on the reaction gas outlet side and with a catalyst for thedehydration reaction of glycerol on the reaction gas inlet side, thusconducting in said one reaction tube the dehydration reaction followedby the gas phase oxidation reaction.
 4. The process for producingacrylic acid according to claim 1 or 3, and further comprising the stepsof: a) adding an inert gas to the first gas to obtain a resultant mixedgas, wherein the inert gas is selected from the group consisting ofnitrogen gas, carbon dioxide gas and rare gas; and b) controlling aconcentration of said inert gas in said resultant mixed gas to aconcentration of not less than 50% by volume.
 5. The process forproducing acrylic acid according to claim 1 or 3, wherein the gas phasein which the dehydration reaction is conducted is a mixed gas comprisingglycerol, water vapor and oxygen.
 6. The process for producing acrylicacid according to claim 5, wherein the amount of the water vaporrelative to the glycerol in the mixed gas is not larger than 1.2 timesby mol.
 7. The process for producing acrylic acid according to claim 1or 3, wherein the acrylic acid is used to produce a water-absorbentresin.
 8. The process for producing acrylic acid according to claim 1 or3, further comprising the step of collecting the resultant acrylic acidas a solution by using water or a solvent.
 9. The process for producingacrylic acid according to claim 8, further comprising a distillationstep for removing low- and high-boiling-point materials from theresultant solution containing acrylic acid.
 10. The process forproducing acrylic acid according to claim 8, further comprising acrystallization step for purifying acrylic acid by crystallizing it. 11.A petroleum independent process for producing acrylic acid, comprisingthe steps of: a) obtaining glycerol from one of i) hydrolysis ofoils-and-fats, ii) waste fluids in soap production, and iii) aby-product in production of a bio-diesel fuel as a renewable fuel; b)vaporizing a raw material comprising an aqueous glycerol solution havingsaid glycerol to generate a first gas, wherein said aqueous glycerolsolution has a water content of not more than 50% by weight; c) addingan inert gas to the first gas to obtain a resultant mixed gas, whereinthe inert gas is selected from the group consisting of nitrogen gas,carbon dioxide gas and rare gas, and controlling a concentration of saidinert gas in said resultant mixed gas to a concentration of not lessthan 50% by volume; d) applying a dehydration reaction to glycerol in agas phase that includes said resultant mixed gas, wherein the gas phasein which said dehydration reaction is conducted comprises glycerol,water vapor and oxygen, and wherein the amount of the water vaporrelative to the glycerol in the mixed gas is not larger than 1.2 timesby mol; then e) applying a gas phase oxidation reaction to a gaseousreaction product formed by the dehydration reaction to obtain saidacrylic acid; and f) wherein the dehydration reaction and the gas phaseoxidation reaction are conducted in a tandem-type reactor comprising tworeaction tubes which are linked to each other, where the two reactiontubes are filled with a catalyst for the dehydration reaction and acatalyst for the gas phase oxidation reaction respectively, and wherethe dehydration reaction and the gas phase oxidation reaction areseparately conducted in their respective tubes.
 12. A process forproducing a water-absorbent resin from glycerol as a raw material,comprising the steps of: producing acrylic acid from glycerol as a rawmaterial by the process according to any one of claims 1 and 11; andthen polymerizing said acrylic acid to obtain a water-absorbent resin.13. A process for producing a water-absorbent resin from acrylic acid asa raw material, comprising the step of polymerizing acrylic acid as araw material to obtain a water-absorbent resin, wherein said acrylicacid is acrylic acid obtained by the process according to any one ofclaims 1 and
 11. 14. A petroleum independent process for producingacrylic acid, comprising the steps of: a) obtaining glycerol from one ofi) hydrolysis of oils-and-fats, ii) waste fluids in soap production, andiii) a by-product in production of a bio-diesel fuel as a renewablefuel; b) vaporizing a raw material comprising an aqueous glycerolsolution having said glycerol to generate a first gas, wherein saidaqueous glycerol solution has a water content of not more than 50% byweight; c) adding an inert gas to the first gas to obtain a resultantmixed gas, wherein the inert gas is selected from the group consistingof nitrogen gas, carbon dioxide gas and rare gas, and controlling aconcentration of said inert gas in said resultant mixed gas to aconcentration of not less than 50% by volume; d) applying a dehydrationreaction to glycerol in a gas phase that includes said resultant mixedgas, wherein the gas phase in which said dehydration reaction isconducted comprises glycerol, water vapor and oxygen, and wherein theamount of the water vapor relative to the glycerol in the mixed gas isnot larger than 1.2 times by mol; then e) applying a gas phase oxidationreaction to a gaseous reaction product formed by the dehydrationreaction to obtain said acrylic acid; and f) wherein the dehydrationreaction and the gas phase oxidation reaction are conducted in asingle-type reactor comprising one reaction tube, where said onereaction tube is filled with a catalyst for the gas phase oxidation onthe reaction gas outlet side and with a catalyst for the dehydrationreaction of glycerol on the reaction gas inlet side, thus conducting insaid one reaction tube the dehydration reaction followed by the gasphase oxidation reaction.
 15. A process for producing a water-absorbentresin from glycerol as a raw material, comprising the steps of:producing acrylic acid from glycerol as a raw material by the processaccording to any one of claims 3 and 14; and then polymerizing saidacrylic acid to obtain a water-absorbent resin.
 16. A process forproducing a water-absorbent resin from acrylic acid as a raw material,comprising the step of polymerizing acrylic acid as a raw material toobtain a water-absorbent resin, wherein said acrylic acid is acrylicacid obtained by the process according to any one of claims 3 and 14.17. The process for producing acrylic acid according to any one ofclaims 1, 3, 11, and 14, wherein said aqueous glycerol solution has awater content of not more than 20% by weight.